Oprah Winfrey isn't afraid to ask tough questions, as Rihanna quickly learned when the 58-year-old TV mogul interviewed her for "Oprah's Next Chapter."

When asked about her former relationship with Chris Brown, who pleaded guilty to assaulting her in February 2009, Rihanna teared up. "It was embarrassing. It was humiliating. I lost my best friend," the 24-year-old pop star said.

Post-split, "I was resentful. I held a grudge," Rihanna admitted. "I was dark." Rihanna and Brown, 23, have since reconciled professionally and, reportedly, personally: The two collaborated on two remixes in early 2012, and a source told Us Weekly they hooked up "on and off" for nearly a year in 2011.

Winfrey admitted she was surprised by Rihanna's tender candor. "I thought she would have been kind of a badass kind of a rocker hard-edge woman. Nothing could have been farther from the truth," she said. "She was thoughtful. She was very emotional. She was vulnerable."